1. Field of the Invention
The present invention relates to a method for treating a spent tin or tin/lead stripping solution used in the electronic industry, particularly in the manufacture of printed circuit boards or lead frames. More specifically, the present invention relates to a method for treating a spent tin or tin/lead stripping solution to recover metals, such as copper, tin, and lead from the solution.
2. Description of the Related Art
In a common printed circuit board manufacturing process, tin or tin/lead is usually electroplated onto a copper layer to serve as an etch-resistance layer. After the non-circuitry regions of the copper layer are etched away, a nitric acid based solution is often used for stripping the electroplated tin or tin/lead layer, thereby exposing the copper circuitry.
The nitric acid based solutions comprise nitric acid and ferric nitrate. When the concentration of free nitric acid is lower than 4N or the concentration of tin is higher than 100 g/L during stripping, it is necessary to replace the spent stripping solution with fresh stripping solution. Normally, the spent stripping solution comprises about 2-15 g/L Cu2+, about 1-40 g/L Fe3+ or Fe2+, about 10-120 g/L Sn4+ or Sn2+, about 0-5 g/L Pb4+ or Pb2+ and about 2.0-6.0N free nitric acid. Since Sn2+ is oxidized to Sn4+ that forms tin oxides or hydroxides, the concentration of Sn2+ in the waste stripping solution is sometimes lower than about 10 g/L during the storage or transportation of the spent stripping solution. Therefore, the spent stripping solution should be further treated for environmental considerations.
The conventional method for treating spent stripping solution is neutralization. For example, sodium hydroxide is added to the spent stripping solution to neutralize the free nitric acid and adjust the pH value to 8-13 such that most metallic cations are converted to metal oxide or hydroxide precipitates. The solid metallic oxides or hydroxides obtained through filtration are subjected to a refining process to recover the metallic tin. The resulting filtrate is then vaporized to produce sodium nitrate crystals.
However, the foregoing method requires use of large amounts of sodium hydroxide, and the resulting sodium nitrate crystals do not possess commercial benefits. Hence, Taiwan Patent Publication No. 177,911, entitled xe2x80x9cA method for recovering metallic tin from spent tin stripping solutions,xe2x80x9d teaches use of a neutralizer, a precipitant and a reducing agent to make the treated effluent meet environmental requirements and to recover metallic tin from the spent stripping solution. However, the process in the ""911 patent still requires use of large amounts of a neutralizer and a reducing agent so the ""911 patent still has economic disadvantages. Further, Taiwan Patent Publication No. 258,758, entitled xe2x80x9cA method and an apparatus for regeneration of tin-electroplating solution,xe2x80x9d describes a method for separating Fe, Cr and Sn ions from the electroplating solution by using ion exchange resins to regenerate tin flux and to recover metallic Sn. However, the method set forth in the ""758 patent involves complex procedures so the method is not useful for treating spent tin/lead stripping solution. Therefore, there is a need for methods for treating a spent tin/lead stripping solution in a cost-effective, simple and efficient manner.
The objective of the present invention is to provide a method for treating a spent tin or tin/lead stripping solution to recover metals, such as copper, tin, and lead from the solution. The method comprises (i) electrolytically reducing copper ions in the solution to copper at a low temperature; (ii) electrolytically oxidizing Sn2+ and Pb2+ in the solution at a high temperature to form solid tin and lead oxides and hydroxides; (iii) separating solid tin and lead oxides and hydroxides from the solution; (iv) dissolving tin and lead oxides and hydroxides obtained in step (iii) in a strong alkali or acidic solution; and (v) electrolytically reducing the alkali or acidic solution obtained in step (iv) at a high temperature to recover metallic tin and lead.
Another advantage of the present invention is that the filtrate obtained in step (iii) of the method is useful for preparing fresh tin or tin/lead stripping solution.